Strategies for acquiring the phospholipid metabolite inositol in pathogenic bacteria, fungi and protozoa: making it and taking it

Abstract

myo-Inositol (inositol) is an essential nutrient that is used for building phosphatidylinositol and its derivatives in eukaryotes and even in some eubacteria such as the mycobacteria. As a consequence, fungal, protozoan and mycobacterial pathogens must be able to acquire inositol in order to proliferate and cause infection in their hosts. There are two primary mechanisms for acquiring inositol. One is to synthesize inositol from glucose 6-phosphate using two sequentially acting enzymes: inositol-3-phosphate synthase (Ino1p) converts glucose 6-phosphate to inositol 3-phosphate, and then inositol monophosphatase (IMPase) dephosphorylates inositol 3-phosphate to generate inositol. The other mechanism is to import inositol from the environment via inositol transporters. Inositol is readily abundant in the bloodstream of mammalian hosts, providing a source from which many pathogens could potentially import inositol. However, despite this abundance of inositol in the host, some pathogens such as the bacterium Mycobacterium tuberculosis and the protist parasite Trypanosoma brucei must be able to make inositol de novo in order to cause disease (M. tuberculosis) or even grow (T. brucei). Other pathogens such as the fungus Candida albicans are equally adept at causing disease by importing inositol or by making it de novo. The role of inositol acquisition in the biology and pathogenesis of the parasite Leishmania and the fungus Cryptococcus are being explored as well. The specific strategies used by these pathogens to acquire inositol while in the host are discussed in relation to each pathogen's unique metabolic requirements.

Fig. 1.

Loss of the MtINO1 gene in Mycobacterium tuberculosis causes a loss of viability and mycothiol in inositol-free media. It is unclear if the Mtino1Δ mutant also experiences a loss of lipoglycans under some growth conditions as has been observed for the Mycobacterium smegmatis Msino1Δ mutant. (a) The Mtino1Δ mutant can only grow like the wild-type strain in medium that contains at least 77 mM inositol, whereas the Msino1Δ mutant only requires 1 mM inositol in the medium for wild-type growth. (b) When the Mtino1Δ mutant is incubated in inositol-free medium there is a decrease in mycothiol levels, but not lipoglycans (PIM, LM and LAM). The Mtino1Δ mutant also does not lose viability under these conditions. Similar results were reported for the Msino1Δ mutant when incubated in inositol-free medium at high density, except that mycothiol was not measured. (c) When the M. smegmatis Msino1Δ mutant is incubated in inositol-free medium at a low dilution (i.e. exponential phase) it experiences a drop in lipoglycan levels and viability. Mycothiol levels were not measured. Yellow hexagon, inositol; green hexagon-P, glucose 6-phosphate; yellow hexagon-P; inositol 3-phosphate; red and yellow phospholipid, PI. These symbols also apply to Figs 2 and 3.

Fig. 2.

The eukaryotic parasite Trypanosoma brucei is able to import inositol from the environment or synthesize it de novo. However, according to the current model, inositol imported from the environment is utilized primarily in bulk phosphatidylinositol (red and yellow phospholipid) production via a phosphatidylinositol synthase (PIS) localized to the Golgi complex. Inositol synthesized de novo is primarily used to generate phosphatidylinositol that is used for production of glycosylphosphatidylinositols (GPIs). The de novo-synthesized inositol is believed to be utilized mostly for GPI production because the IMPase that dephosphorylates inositol 3-phosphate to inositol is localized to the ER, where GPI synthesis occurs. Mutants lacking TbINO1 are inviable because of diminished GPI production.

Fig. 3.

The fungal pathogen Candida albicans can acquire inositol by de novo synthesis or by importing it, and either mechanism is sufficient to support wild-type growth in vitro and full virulence.